Turbine wheel



C. J. M DOWALL TURBINE WHEEL 5 Sheets-Sheet 1 Filed Feb. 19. 1952 attorneys Oct. 21, 1958 Filed Feb. 19, 1952 C. J. M DOWALL TURBINE WHEEL I 5 Sheets-Sheet 2 Oct. 21, 1958 c. J. M DOWALL 2,357,132

TURBINE WHEEL Filed Feb. 19, 1952 3 Sheets-Sheet 25 SP/AAL 7/7695/105 P/TCH OF THE INVEVTOR.

ggazzs Jiicflozm)? A 77' ORNE Y United States Patent TURBINE WHEEL Charles J. McDowall, Indianapolis, Ind., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application February 19, 1952, Serial No. 272,467

Claims. (Cl. 253-3915) My invention relates primarily to turbine wheel assemblies and, more particularly, to the mounting of blades on turbine wheels and other structures of similar character. The preferred embodiment of the invention is directed to a turbine wheel or rotor stage of a gas turbine engine of a character suited for aircraft appiications, but the features of the invention may be applied in other environments.

A great variety of arrangements for mounting blades or buckets on turbine wheels have been proposed. The standard conventional blade mounting at present is commonly known as the fir tree root. In this mounting the blades are mounted in V-shaped slots extending across the rim of the turbine wheel, the slots being formed with a number of grooves which are engaged by corresponding ribs on the generally V-shaped base or root of the turbine bucket.

The present invention is directed to provision for re taining the blades by circumferential spiral grooves or threads in the turbine wheel, so that the blades are assembled and placed by sliding them around the circumference of the wheel rather than across the rim. Blade mountings of this general type have also been proposed hitherto. to elimination of undesirable characteristics of previous proposals for mounting blades in this manner. A feature of the invention is that the lead of the mounting grooves or threads is such that all the blade roots maybe identical and that a broken or defective blade may be readily removed by releasing only a few adjacent blades. The invention is further characterized by very simple and effective means for spacing the blades which is easily carried into effect.

A further advantage of the invention lies in improved provision for circulation of a cooling medium such as air to the buckets, particularly in a wheel having a blade mounting of the character described. This is one of the advantages of the present invention over the conventional fir tree blade mounting, which is not well adapted to circulation of air through the blades.

The advantages of the invention will be clear to those skilled in the art from the succeeding detailed description of the preferred embodiment of the invention in a turbine rotor stage and the accompanying drawings, in which Fig. 1 is a partial axial sectional view of a single stage gas turbine, the section being taken on a plane containing the axis of rotation of the turbine; Fig. 2 is a fragmentary elevation view of the turbine wheel in the direction indicated by the plane 2-2 of Fig. 1, with some of the parts removed; Fig. 3 is a developed plan The present invention, however, is directed view of the rim of the wheel, partly in section and with certain parts removed, taken on the surface indicated by the line 3-3 of Fig. 2; Fig. 4 is a radial section taken on the plane 4-4 of Fig. 2; Fig. 5 is a partial enlarged sectional view taken on the plane 5-5 of Fig. 4; Fig. 6 is a partial radial section of an alternative form of turbine wheel; and Figure 7 is a diagram illustrating the meaning of terms used in the specification.

Referring to Fig. l, which illustrates the turbine .structure other than the rotor stage of the invention more -or less diagrammatically, the turbine wheel 10 is mounted 2,857,132 Patented Oct. 21, 19,58

for rotation on a shaft (not shown) in any suitable fashion. Blades or buckets 11 extending from the rim of the wheel are mounted in an annular passage for motive fluid. The turbine is mounted within a casing 12 fixed to a nozzle ring 13 comprising an outer shroud 14, an inner shroud 16, and nozzle vanes 17. Motive fluid is supplied to the nozzle ring from any suitable source, such as the combustion chambers (not shown) of a gas turbine engine and is directed by the vanes 17 against the buckets 11. The motive fluid is discharged into an annular exhaust duct defined by an outer wall or tailpipe 18 and an inner wall or tailcone 19. The structure indicated at 21 extending inwardly from the nozzle is part of the fixed frame of the engine in which the bearings for the turbine wheel shaft may be mounted and which provides a support for the turbine nozzle. Since the fixed structure of the engine may be of any suitable type, the details thereof are immaterial to the invention. It is deemed unnecessary to discuss these further.

The invention is concerned with the rotor stage comprising the wheel 10 and buckets 11. The wheel 10 comprises a disk 23 and a rim which flares outwardly and is divided into two parts 24 and 26 which define between them an annular air chamber or air passage 27. The outermost portion of the rim tapers inwardly and is provided with generally spiral grooves or threads 28, there being one set of such grooves on each of the parts 24 and "26, the tooth form of which is preferably similar to the form of the dovetail lands and grooves of the conventional fir tree blade mounting. The bucket 11 is preferably a single casting comprising three principal parts, the blade 30, the blade platform 31 at the base of the blade, and the bifurcated root 32 extending in wardly from the blade platform. The two forks of the blade root are formed internally with generally spiral threads or grooves which mate with the threads 28 of the wheel, the lands of the roots entering the grooves of the wheel. As will be seen, each bucket can be mounted on the wheel by sliding it circumferentially until it seats against the rim of the wheel, or approximately to this position. Some looseness of fit is ordinarily provided in blade mountings, in view of the thermal expansion of the parts and because some looseness of fit may be beneficial in damping vibration of the blades.

The threads 28 are interrupted by radial grooves or slots 34 in the faces of the wheel rim, these slots being located intermediate the blade positions. Slots 34 are provided for the mounting of spacer keys 36 inserted between the blade roots to determine equal angular spacing of the blades. The keys 36 are inserted after the blades are in place and may be removed in order to remove a blade. Each key 36 comprises a generally rectangular body 37 which tapers slightly from the outer end of the key to the inner end, as will be apparent from Fig. 4. Approximately half of the body 37 seats in the slot 34 of the rim and approximately half is seated in notches 38 in the edges of the blade platforms and roots. The key 36 also preferably comprises a rib 39 which extends forwardly or rearwardly from the rim of the wheel between the opposed lateral faces of the blade roots. Thus, with two adjacent blades in position, the keys 36 may be inserted into the slots 3d between the blade roots at each face of the rim, positively locating the blades in position.

Since the inwardly facing surface of the notch 38 converges toward the inner surface of the groove 34 as the axis of the wheel is approached, the keys 36 cannot fall out of the slots in a radially inwardly direction. In the operation of the turbine there is, of course, a heavy centrifugal force on the keys tending to hurl them radially outward from the wheel. The keys 36 are held against this centrifugal force by a sealing strip 41 extending across the rim of the wheel, which also serves to close the spaces between the blade platforms 31 against undue leakage of air from the chamber 27. The sealing strips 41 are aligned with the keys 36 and extend across the outer ends of the aligned keys at each face of the wheel. These strips are disposed under shoulders 42 (Fig. 2) defined by grooves in the axially extending faces of the blade platforms. These strips may be retained in any convenient manner. Preferably, as illustrated, the ends of the strip are bent over against the outer faces of the blade root and the outer surface of the portion 39 of the key, as indicated at 43.

While the blade mounting structure of the invention may be employed in wheels in which there is no provision for air cooling, the invention is particularly adapted to the provision of such cooling. As illustrated, the buckets 11 are formed with internal passages 44 extending from the base through the entire length of the blade, so that air may be circulated through the blade and discharged from the tip into the motive fluid passage. This air flows from the chamber 27 through each blade. The invention is not particularly concerned with the arrangements for supplying cooling air to the chamber 27, and any suitable structure may be employed. As illustrated in Figs. 1 to 5, passages 46 may be provided in the rim portion 24 of the wheel into which cooling air may be supplied in known manner from between the fixed structure 21 and the disk 23. Alternatively, as illustrated in Fig. 6, the disk 23' of a modified wheel may be formed with radial passages 47 leading from the axis of the wheel or adjacent thereto to the chamber 27. The passages 47 may be provided in known manner by forming the disk 23' of two separate disks fixed together or may be drilled passages, if desired.

A' significant feature of the invention is that the ratio of the lead of the threads 28 to the pitch of the threads is equal to the number of turbine buckets on the wheel, so that the radial spacing of the grooves 28 from the rim of the wheel is the same at each blade position. Thus, all the buckets may be cast or machined to provide roots of identical form and all the blades are interchangeable.

The manner of assembling and disassembling the rotor structure will presumably be clear to those skilled in the art, but it may be described briefly. The buckets 10 are all slid circumferentially of the rotor approximately to final position. Because of the very considerable lead of the threads 28 and the clearance between the blade platforms, it would be possible to mount all of the buckets even though the rim of the wheel were not tapered. However, the conical configuration of the surface in which the threads are generated, as opposed to a true radial or plane surface, makes it possible to seat the blades with a rather small amount of circumferential motion. When the buckets are all in place, the keys 36 are inserted and then the sealing strips 41 are slid into place. One end of the strip may be bent over before insertion and the other is bent after the strip is in place. It will be seen that the keys 36 insure accurate spacing of the blades and that the strips 41, in addition to retaining the keys, seal off the chamber 27 so that the air, apart from minor leakages, is directed to the passages 44 in the blades.

It is not necessary to have all the buckets in place before the keys are inserted, but the first few must be left free for circumferential movement until the last bucket is in place. In order to remove a damaged bucket, it is necessary only to remove the sealing strips 41 and keys 36 behind that bucket and several of the buckets immediately behind the defective one, so that the adjoining buckets may be slid together sufiiciently to permit the defective bucket to be slid sufliciently to release it from the threads. The replacement bucket may then be inserted in the same manner as previously outlined. The keys may be reused but new sealing strips should be substituted for those removed.

It is apparent that modifications of this structure embodying some or all of the principles and features of the invention may be made within the scope of the invention. Particularly, it may be noted that, instead of the blade root embracing the wheel, the wheel may embrace the blade root. In other words, the wheel may be internally threaded and the blade root externally threaded. It is not essential to the employment of the invention, although it is highly desirable both for reasons of structural strength and for facilitating assembly and disassembly, that the surface in which the threads are generated be conical rather than plane.

The invention may, obviously, be employed without the inclusion of provisions of blade cooling.

While it is desirable that the lead of the threads equal the product of the pitch of the threads by the number of blades so that all of the blade roots will be identical, the advantages of the invention will be realized to some extent and certain other advantages may be gained by providing such a relation between these three factors that the blades fall into a small number of groups such as two or three groups, with identical roots on all the blades of each group. For example, if the lead of the threads equals one-half of or one and a half times the product of the pitch of the threads by the number of the blades, the blades will fall into two groups.

The above may be clarified by reference to Figure 7 which shows diagrammatically a set of spiral threads, specifically one having six starts. The lead of the thread, as indicated, is the advance of a particular thread or groove in a complete rotation about the axis. The pitch of the thread is the distance from the crest of one thread to the crest of the adjacent thread. Thus, with six starts as shown in this diagram, the lead is six times the pitch. It may be pointed out in connection with this illustrative six start thread that, if six equally spaced blades or buckets were installed, they would all have identical roots and would constitute a single group. If twelve equally spaced blades or four equally spaced blades were mounted on the wheel with a six start thread, there would be two groups of blades with the blades in each group having identical roots.

Many other variations in the structure may be made within the scope of the invention, which is not to be regarded as limited by the detailed description of the preferred embodiment.

I claim:

1. A turbine rotor stage comprising, in combination, a wheel, a number of circumferentially spaced hollow blades thereon, the wheel and blades being formed with spiral threads cooperating to provide a dovetail attachment for the blades, the rim of the wheel defining a cooling air passage open at the periphery of the wheel adapted to communicate with the hollow blades, and strips extending across the rim of the wheel and extending between adjacent blades to seal the passage at the spaces between the blades.

2. A turbine rotor stage comprising, in combination, a wheel, a. number of circumferentially spaced hollow blades thereon, the wheel and blades being formed with spiral threads cooperating to provide a dovetail attachment for the blades, the blades having attaching portions embracing the wheel rim, the rim of the wheel defining a cooling air passage open at the periphery of the wheel adapted to communicate with the hollow blades, and strips extending across the rim of the wheel and extending between adjacent blades to seal the passage at the spaces between the blades.

3. A turbine rotor stage comprising, in combination, a Wheel and a number of buckets thereon, the wheel and buckets having spiral threads cooperating to provide a dovetail attachment for the buckets, the lead of the threads equalling the pitch of the threads multiplied by the number of buckets.

4. A turbine rotor stage comprising, in combination, a

wheel, a number of buckets thereon, the wheel and buckets having spiral threads cooperating to provide a dovetail attachment for the buckets, the wheel having generally radial grooves therein, spacer keys inserted in the said grooves in the wheel between the buckets, strips extending across the rim of the wheel over the outer ends of the keys, and means on the buckets for retaining the strips.

5. A turbine rotor stage comprising, in combination, a Wheel, a number of buckets thereon, the wheel and buckets having spiral threads cooperating to provide a dovetail attachment for the buckets, the wheel having generally radial grooves therein, spacer keys inserted in the said grooves in the wheel between the buckets and retained by the buckets, strips extending across the rim of the wheel over the outer ends of the keys, and means on the buckets for retaining the strips.

6. A turbine rotor stage comprising, in combination, a wheel, a number of buckets thereon, the wheel and buckets having spiral threads cooperating to provide a dovetail attachment for the buckets, the attaching portion of the buckets embracing the wheel rim, the wheel having generally radial grooves therein, spacer keys inserted in the said radial grooves in the wheel between the buckets and retained by the buckets, strips extending across the rim of the wheel over the outer ends of the keys, and means on the buckets for retaining the strips.

7. A turbine rotor stage comprising, in combination, a wheel, a number of buckets thereon, the wheel and buckets having circumferential lands and grooves cooperating to provide a dovetail attachment for the buckets, the wheel having generally radial grooves therein, spacer keys inserted in the said radial grooves in the wheel between the buckets and retained by the buckets, strips extending across the rim of the wheel over the outer ends of the keys, and means on the buckets for retaining the strips.

8. A turbine rotor stage comprising, in combination, a wheel formed with two sets of spiral grooves for blade attachment, a number of blades disposed around the circumference of the wheel and circumferentially spaced thereon and having roots formed with spiral lands engaging in the spiral grooves, the wheel having generally radial slots therein, means engaging the blade roots for maintaining the desired circumferential spacing of the blades on the wheel comprising generally radially extending keys mounted in the said slots in the wheel between the blade roots, and strips extending across the rim of the Wheel and over the outer ends of the keys for retaining the keys, the strips being held radially by shoulders on the blade roots.

9. A turbine rotor stage comprising, in combination, a wheel formed with two sets of spiral grooves for blade attachment, a number of blades disposed around the circumference of the wheel and circumferentially spaced thereon and having roots formed with spiral lands engaging in the spiral grooves, the wheel having generally radial slots therein, means engaging the blade roots for maintaining the desired circumferential spacing of the blades on the wheel comprising generally radially extending keys mounted in the said slots in the wheel between the blade roots, and strips extending across the rim of the wheel and over the outer ends of the keys for retaining the keys, the strips being held radially by shoulders on the blade roots and being bent over at the ends to prevent displacement longitudinally of the strips.

10. A turbine rotor stage comprising, in combination, a wheel, a number of circumferentially spaced hollow blades thereon, the wheel and blades being formed with spiral threads cooperating to provide a dovetail attachment for the blades, the rim of the wheel defining a cooling air passage open at the periphery of the wheel adapted to communicate with the hollow blades, spacer keys inserted generally radially of the wheel between the blades, and strips extending across the rim of the wheel and extending between adjacent blades to seal the passage at the spaces between the blades, the strips extending over the outer ends of the spacer keys.

11. A turbine rotor stage comprising, in combination, a wheel formed with two sets of spiral treads for blade attachment, a number of circumferentially spaced blades having roots formed with spiral threads engaging in the wheel threads, the lead of the threads being so related to the pitch of the threads and the circumferential spacing of the blades that the totality of blades consists of a small number of groups such as two or three groups, with the threads of the roots of the blades in each group identical.

12. A turbine rotor stage comprising, in combination, a wheel formed with two sets of spiral threads for blade attachment, a number of circumferentially spaced blades having roots formed with spiral threads engaging in the wheel threads, the lead of the threads being so related to the pitch of the threads and the circumferential spacing of the blades that the threads of the roots are identical, and means cooperating with the wheel and blades for maintaining the desired circumferential spacing of the blades on the wheel.

13. A turbine rotor stage comprising, in combination, a wheel, a number of circumferentially spaced hollow blades thereon, the wheel and blades having circumferential lands and grooves cooperating to provide a dovetail attachment for the blades, the rim of the wheel defining a cooling air passage open at the periphery of the wheel adapted to communicate with the hollow blades, and strips extending across the rim of the wheel and extending between adjacent blades to seal the passage at the spaces between the blades, the blades defining shoulders overlying and spaced from the wheel rim and the strips being slidable in a direction across the rim into the spaces between the blades and the wheel rim for assembly thereof.

14. A turbine rotor stage comprising, in combination, a wheel formed with two sets of spiral threads for blade attachment, a number of circumferentially spaced blades having roots formed with spiral threads engaging in the wheel threads and uniformly circumferentially spaced on the Wheel, the ratio of the lead of the threads to the pitch of the threads being an integral multiple of the number of blade roots so that the threads of the blade roots are identical.

15. A turbine rotor stage comprising, in combination, a wheel formed with two sets of spiral threads for blade attachment, a number of blades disposed around the circumference of the wheel and circumferentially spaced thereon and having roots formed with spiral threads engaging in the wheel threads, the lead of the threads being so related to the pitch of the threads and the circumferential spacing of the blades that the threads of the roots are identical, the wheel having generally radial slots therein, means engaging the blade roots for maintaining the desired circumferential spacing of the blades on the wheel comprising generally radially extending keys mount ed in the said slots in the wheel between the blade roots, and strips extending across the rim of the wheel and over the outer ends of the keys for retaining the keys, the blade roots having inwardly facing shoulders and the strips being held radially by the shoulders on the blade roots.

References Cited in the file of this patent UNITED STATES PATENTS France Sept. 2, 1929 

